Free-piston compressor



Fatenie .Fan. l,

FREE-PISTON COMPRESSOR Andrew Kalitinsky, New York, N. Y., assignor to United Aircraft Corporation, East Hartford, Conn, a corporation of Delaware Application June 27, 1946, Serial No. 679,798

Claims. I

This invention relates to free-piston compressors and particularly to an arrangement by which to run-in a high-speed compressor at a relatively low speed.

It is well known that the frequency of the cycle in free-piston units is dependent upon the pressures on the piston assemblies and also dependent upon the weight of the piston assemblies. If a free-piston unit is made with a lightweight piston assembly for the purpose of highspeed operation the unit is incapable of operating at the relatively low speeds desirable for breaking in the wearing surfaces of the unit. A feature of this invention is the substitution of a relatively heavy piston assembly for the regular piston assembly for the purpose of causing the unit to operate during the run-in period at relatively slow speeds. 1

In the conventional type of engine having a crankshaft the range of speeds over which the engine will operate is relatively large and it is possible to idle the engine at speeds satisfactory for running-in the engine. In the free-piston units. however, the absence of the crankshaft and the narrow range of speed between minimum and maximum speeds makes it impossible to operate the unit at the slow speeds advantageous for initial operation of the unit. A feature of the invention is an arrangement for increasing the moving mass of the unit for the purpose of substantially reducing the rate of piston reciprocation during the initial run-in of the unit. It will be understood that this weight increase is not necessarily obtained by increasing the weight of the piston itself since it may result from an increase in any part of the oscillating mass which forms a part of the synchronizing mechanism between the opposed piston assemblies.

Other objects and advantages will be apparent from the specification and claims, and from the accompanying drawings which illustrate an embodiment of the invention.

Fig. l is a section view through the free-piston unit.

Fig. 2 is a fragmentary sectional view through a piston assembly having a much greater mass.

The'unit shown includes an engine cylinder l0 having reciprocating pistons l2 and I4 to which compressor pistons l6 and It in cylinders and 22 are integrally connected. Sleeves 24 and 26 attached to the compressor pistons complete the reciprocating pistonassemblies. The sleeves in combination with stationary pistons 26 and 30 form airspring cylinders.

The piston assemblies are moved apart by the burning of fuel injected into the engine cylinder l0 through one or more nozzles 32. Air compressed in the airspring cylinders on the power stroke returns the piston assemblies. The assemblies are always maintained at equal distances from the center of the engine cylinders by a linkage which may include racks 34 and 36 extending from the opposed piston assemblies and meshing with a pinion 38 on a shaft 40.

Intake manifold 42 which extends around the compressor and engine cylinders conducts air to intake valves 44 in the heads of the compressor cylinders through which air alternately enters opposite ends of these cylinders. The compressed air leaves the cylinders through discharge valves 46 also by opposite ends of the compressor cylinders and passes into a center scavenge chamber 48 and end chambers 50 and 52. These chambers may be connected by a scavenge manifold, not shown.

Compressed gas from the chambers enters engine ports 54 and 56 which are uncovered by pistons l2 and I 4 at the end of the power stroke thereby permitting air to be blown through the engine cylinder. Gas in the engine cylinders is discharged through exhaust ports 58 into exhaust manifolds 60.

One of the principal limitations on the frequency or speed of a free-piston unit is the mass of the moving parts which include not only the piston assemblies as above described but also the synchronizing racks and pinion and any other mechanism actuated by the racks or pinion. For the purpose of providing a unit operating at a relatively high frequency, the engine pistons l4 are made hollow as shown and the compressor pistons l6 and I8 may also be made hollow for the purpose of further reducing the weight of the piston assemblies. Thus, for example if the weight of the piston assemblies can be reduced enough the full load frequency of the unit may be, for example, as high as three thousand cycles per minute in which case the minimum frequency may be not lower than two thousand cycles. In many cases this minimum frequency is still too high for satisfactory run-in of a new unit. In order to reduce the speed or frequency of the unit the moving mass may be substantially increased in weight as, for example, as shown in Fig. 2 by inserting a solid central element 62 in the engine piston or by substituting a solid compressor piston 64 for the regular compressor piston or both. Either of these arrangements for increasing the weight does not affect the external dimensions of the piston assemblies and the same outside shell 68 of the engine piston and the same sleeve 26 for the air spring may be retained during the run-in period. An increase in the weight of the piston assembly, as provided above, will reduce the frequency of the unit, for example to as low as a thousand cycles so that the unit may be run-in at a much more advantageous speed. It will be noted that the element 62 may be mounted releasably in place by means of threads 61 on the piston assembly.

Instead of increasing the weight of the piston assembly as such, it may be advantageous, as shown in the dotted lines in Fig. 1, to mount a heavy mass 68 releasably on the end of the shaft I on which the synchronizing pinion is fixed. This mass 58 will effectively reduce the frequency of the unit without any change in the essential dimensions of the unit assembly and permits the unit to be runin with the same piston assemblies that will be used during the normal operation of the unit. The mass 68 may be removed after the run-in and the unit will then operate at the design speed.

It is to be understood that the invention is not limited to the specific embodiment herein illustrated and described, but may be used in other ways without departure from its spirit as defined by the following claims.

I claim.

1. In a high-speed free-piston unit in which the rate of reciprocation of the piston assembly is an inverse function of the reciprocating mass, the method of running-in a unit which involves adding a mass internally of the piston assembly to obtain a slow rate of piston reciprocation without change in the external dimensions of the piston assembly and subsequently removing the added mass to obtain a normal rate of piston reciprocation.

2. In a highspeed free-piston unit in which the piston assembly is moved alternately in opposite directtions by gas forces on the faces of the piston elements of the assembly, the method of running-in a unit at slow speed, which involves substantially increasing the weight of the piston assembly without a change in the essential dimensions thereof for temporarily reducing the normal frequency of the piston reciprocation during the run-in period, and subsequently removing the increase in weight to obtain a normal rate of piston reciprocation.

3. In a high-speed free-piston unit having a moving mass including opposed piston assemblies and a synchronizing mechanism interconnecting the assemblies, the method of runningin a unit which involves substantially increasing the weight of the moving mass without affecting the external dimensions of the piston assemblies thereby effectively reducing temporarily the frequency of the piston reciprocation durin the run-in period, and subsequently removing the increase in weight to obtain a normal rate of piston reciprocation.

4. A high-speed free-piston compressor including opposed piston assemblies, and a synchronizing mechanism interconnecting the assemblies, in combination with means located internally of the piston assemblies for removably attachin within each of the assemblies an additional mass for the purpose of temporarily reducing the rate of piston reciprocation, said additional masses being removable to provide for a normal rate of piston reciprocation after the run-in period.

5. A high-speed free-piston compressor having a moving mass including opposed piston assemblies, and a synchronizing mechanism interconnecting the assemblies, in combination with additional weights, means for releasably attaching to said moving mass the additional weights for reducing the rate of piston reciprocation during the run-in period, said additional weights being removable to provide for a normal rate of piston reciprocation after the run-in period.

ANDREW KALITINSKY.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,722,425 Junkers July 30, 1929 1,747,948 Pescara Feb. 18, 1930 

